Larval drift and development of amphidromous fishes, particularly the bluegill bully (Gobiomorphus hubbsi)

Amphidromy is a distinct life history strategy found in many aquatic organisms, involving a return migration (‘drift’) to a pelagic feeding habitat (usually the sea), undertaken by newly hatched larvae. The freshwater fish faunas of many Indo-Pacific islands are dominated by amphidromous species, yet they remain understudied, especially their larval stages. Amphidromous larvae hatch out exceptionally small and undeveloped, and so face a range of specific challenges during migration such as irreversible starvation and failed development if migration is delayed, as well as management difficulties due to their small size. Basic ecological knowledge such as timing and extent of migration remains unknown, but is crucial to the management of amphidromous species. It was therefore the aim of this thesis to further our knowledge on the larval ecology and migration of a number of New Zealand’s amphidromous fish species. This thesis examines patterns of larval drift and development, focussing on the bluegill bully (Gobiomorphus hubbsi), an endemic eleotrid. A distinct diel and spatial drift pattern was documented, with the vast majority of fish larvae migrating to sea within a few hours of sunset. It is suggested that targeting conservation measures within this window of drift represents a potentially beneficial management strategy for amphidromous species. Development and starvation of larvae was also examined, both through field studies and lab experiments. No distinct pattern of starvation was observed in larvae during their seaward migration, however larvae retained in freshwater failed to develop to as complete a state as those transferred to seawater, implying delayed migration may adversely affect amphidromous fishes developmentally, ultimately reducing their success upon reaching the sea. These results indicate both threats to amphidromous fishes during their larval migration, and a potential approach which may prove beneficial in conserving these fascinating and vulnerable species